Re-enable all tests

Summary:
documenttest: Port away from obsolete API.
documenttest: only compile when KDcraw is found
thumbnailprovidertest: Port to QUrl.
thumbnailprovidertest, documenttest: count remote tests as skipped when unavailable, instead of silently ignoring them (also: no more SkipSingle in Qt5)

Depends on D5749

Test Plan:
documenttest:
- Run documenttest: no failures, 5 skipped tests.
- With GV_REMOTE_TESTS_BASE_URL set: ./gwenview-remote-tests/test.png created on remote host, no failures, 3 skipped tests.
- Execute ./fetch_testing_raw.sh: no failures, 0 skipped tests.
- Uninstall libkdcraw-devel: cmake mentions optional package not being found, documenttest not compiled.

thumbnailprovidertest:
- Run thumbnailprovidertest: no failures, 1 skipped test.
- With GV_REMOTE_TESTS_BASE_URL set: 0 skipped tests.

Reviewers: ltoscano, sandsmark, gateau

Reviewed By: ltoscano

Differential Revision: https://phabricator.kde.org/D5751